Multi-gate nor flash thin-film transistor strings arranged in stacked horizontal active strips with vertical control gates

ABSTRACT

Multi-gate NOR flash thin-film transistor (TFT) string arrays (“multi-gate NOR string arrays”) are organized as stacks of horizontal active strips running parallel to the surface of a silicon substrate, with the TFTs in each stack being controlled by vertical local word-lines provided along one or both sidewalls of the stack of active strips. Each active strip includes at least a channel layer formed between two shared source or drain layers. Data storage in the TFTs of an active strip is provided by charge-storage elements provided between the active strip and the control gates provided by the adjacent local word-lines. Each active strip may provide TFTs that belong to one or two NOR strings, depending on whether one or both sides of the active strip are used.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/220,375, filed Jul. 26, 2016, entitled “Multi-Gate NOR FlashThin-Film Transistor Strings Arranged In Stacked Horizontal ActiveStrips With Vertical Control Gates,” which relates to and claimspriority of U.S. provisional patent application Ser. No. 62/235,322,filed Sep. 30, 2015, entitled “Multi-Gate NOR Flash Thin-Film TransistorStrings Arranged In Stacked Horizontal Active Strips With VerticalControl Gates.” Both Applications are hereby incorporated by referencein their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high-density memory structures. Inparticular, the present invention relates to high-density memorystructures formed by interconnected thin-film storage elements, such asthin-film storage transistors.

2. Discussion of the Related Art

In this disclosure, memory circuit structures are described. Thesestructures may be fabricated on planar semiconductor substrates (e.g.,silicon wafers) using conventional fabrication processes. To facilitateclarity in this description, the term “vertical” refers to the directionperpendicular to the surface of a semiconductor substrate, and the term“horizontal” refers to any direction that is parallel to the surface ofthat semiconductor substrate.

A number of high-density non-volatile memory structures, sometimesreferred to as “three-dimensional vertical NAND strings,” are known inthe prior art. Many of these high-density memory structures are formedusing thin-film storage transistors formed out of deposited thin-films(e.g., polysilicon thin-films), and organized as arrays of “memorystrings.” One type of memory strings is referred to as NAND memorystrings or simply “NAND strings”. A NAND string consists of a number ofseries-connected storage transistors (“TFTs”). Reading or programmingany of the series-connected TFTs requires activation of allseries-connected TFTs in the NAND string. Under this NAND arrangement,the activated TFTs that are not read or programmed may experienceundesirable program-disturb or read-disturb conditions. Further, TFTsformed out of polysilicon thin films have much lower channelmobility—and therefore higher resistivity—than conventional transistorsformed in a single-crystal silicon substrate. The higher seriesresistance in the NAND string limits the number of TFTs in a string inpractice to typically no more than 64 or 128 TFTs. The low read currentthat is required to be conducted through a long NAND string results in along latency.

Another type of high density memory structures is referred to as the NORmemory strings or “NOR strings.” A NOR string includes a number ofstorage transistors each connected to a shared source region and ashared drain region. Thus, the transistors in a NOR string are connectedin parallel, so that a read current in a NOR string is conducted over amuch lesser resistance than the read current through a NAND string. Atthe present time, the present inventor is not aware of any NOR string inthe prior art that is formed out of TFTs. To read or program a storagetransistor in a NOR string, only that storage transistor needs to beactivated (i.e., “on” or conducting), all other storage transistors inthe NOR string remain dormant (i.e., “off” or non-conducting).Consequently, a NOR string allows much faster sensing of the activatedstorage transistor to be read and avoids program-disturb or read-disturbconditions in the other storage transistors of the NOR string that arenot read or programmed.

Three-dimensional memory structures are disclosed, for example, in U.S.Pat. No. 8,878,278 to Alsmeier et al. (“Alsmeier”), entitled “CompactThree Dimensional Vertical NAND and Method of Making Thereof,” filed onJan. 30, 2013 and issued on Nov. 4, 2014. Alsmeier discloses varioustypes of high-density NAND memory structures, such as “terabit cellarray transistor” (TCAT) NAND arrays (FIG. 1A), “pipe-shaped bit-costscalable” (P-BiCS) flash memory (FIG. 1B) and a “vertical NAND” memorystring structure. Likewise, U.S. Pat. No. 7,005,350 to Walker et al.(“Walker I”), entitled “Method for Fabricating Programmable Memory ArrayStructures Incorporating Series-Connected Transistor Strings,” filed onDec. 31, 2002 and issued on Feb. 28, 2006, also discloses a number ofthree-dimensional high-density NAND memory structures.

U.S. Pat. No. 7,612,411 to Walker (“Walker II”), entitled “Dual-GateDevice and Method” filed on Aug. 3, 2005 and issued on Nov. 3, 2009,discloses a “dual gate” memory structure, in which a common activeregion serves independently controlled storage elements in two NANDstrings formed on opposite sides of the common active region.

U.S. Pat. No. 6,744,094 to Forbes (“Forbes”), entitled “Floating GateTransistor with Horizontal Gate Layers Stacked Next to Vertical Body”filed on Aug. 24, 2001 and issued on Jun. 1, 2004, discloses memorystructures having vertical body transistors with adjacent parallelhorizontal gate layers.

U.S. Pat. No. 6,580,124 to Cleaves et al, entitled “MultigateSemiconductor Device with Vertical Channel Current and Method ofFabrication” filed on Aug. 14, 2000 and issued on Jun. 17, 2003,discloses a multibit memory transistor with two or four charge storagemediums formed along vertical surfaces of the transistor.

A three-dimensional memory structure, including horizontal NAND stringsthat are controlled by vertical polysilicon gates, is disclosed in thearticle “Multi-layered Vertical gate NAND Flash Overcoming StackingLimit for Terabit Density Storage” (“Kim”), by W. Kim at al., publishedin the 2009 Symposium on VLSI Tech. Dig. Of Technical Papers, pp188-189. Horizontal 3D NAND strings with vertical poly gates. Anotherthree-dimensional memory structure, also including horizontal NANDstrings with vertical polysilicon gates, is disclosed in the article, “AHighly Scalable 8-Layer 3D Vertical-gate (VG) TFT NAND Flash UsingJunction-Free Buried Channel BE-SONOS Device,” by H. T. Lue et al.,published in the 2010 Symposium on VLSI: Tech. Dig. Of Technical Papers,pp.131-132.

In the memory structures discussed herein, stored information isrepresented by the stored electric charge, which may be introduced usingany of a variety of techniques. For example, U.S. Pat. No. 5,768,192 toEitan, entitled “Non-Volatile Semiconductor Memory Cell UtilizingAsymmetrical Charge Trapping,” filed on Jul. 23, 1996 and issued on Jun.16, 1998, discloses NROM type memory transistor operation based on a“hot electron channel injection” technique. Other techniques includeFowler-Nordheim tunneling used in TFT NAND strings, and directtunneling, both of which are known to those of ordinary skill in theart.

SUMMARY

According to one embodiment of the present invention, multi-gate NORflash thin-film transistor (TFT) string arrays (“multi-gate NOR stringarrays”) are organized as stacks of horizontal active strips runningparallel to the surface of a silicon substrate, with the TFTs in eachstack being controlled by vertical local word-lines provided along oneor both sidewalls of the stack of active strips. Each active stripincludes at least a channel layer formed between two shared source ordrain layers. Data storage in the TFTs of an active strip is provided bycharge-storage elements provided between the active strip and thecontrol gates provided by the adjacent local word-lines. Each activestrip may provide TFTs that belong to one or two NOR strings, dependingon whether one or both sides of the active strip are used.

In one embodiment, only one of the shared source or drain layers in anactive strip is connected to a supply voltage, while the other source ordrain layer is held at a voltage determined by a quantity of chargedeposited in the source or drain layer. Prior to a read, write or eraseoperation, the TFTs that are not to be activated act as a stripcapacitor, with one plate being the source or drain layer itself and theother plate being the control gate electrodes in the NOR string that arereferenced to a ground reference. The charge on the strip capacitor isprovided by one or more pre-charge TFTs that are activated momentarilyto transfer charge to the strip capacitor from a supply voltageconnected to the contacted source or drain layer.

In one embodiment, TFTs are formed on both vertical side edges of eachactive strip, so that vertical local word-lines may be provided alongboth vertical side edges of the active strips. In that embodiment,double-density is achieved by having the local word-lines along one ofvertical edges of an active strip be contacted by horizontal globalword-lines provided above the active strip, while the local word-linesalong the other vertical edge of the active strip be contacted byhorizontal global word-lines provided beneath the active strip. Allglobal word-lines may run in a direction perpendicular to the directionof the corresponding active strips. Even greater storage density may beachieved by storing more than one bit of data in each TFT.

Organizing the TFTs into NOR strings—rather than the prior art NANDstrings—results in (i) a reduced read-latency that approaches that of adynamic random access memory (DRAM) array, (ii) reduced sensitivities toread-disturb and program-disturb conditions that are associated withlong NAND strings, and (iii) reduced power dissipation and lower costs,relative to planar NAND or 3D NAND arrays.

According to one embodiment of the present invention, variations inthreshold voltages within a block of NOR strings may be compensated byproviding electrically programmable reference strings within the block.Effects on a read operation due to background leakage currents inherentto multi-gate NOR strings can be substantially eliminated by comparingthe sensed result of the TFT being read and that of a concurrently readTFT on a reference NOR string. In other embodiments, the charge-storingelement of each TFT may have its structure modified to provide a highwrite/erase cycle endurance, albeit a lower retention time that requiresrefreshing. However, as such refreshing is required significantly lessfrequently than a conventional dynamic random access memory (DRAM)circuit, the NOR string arrays of the present invention may operate insome DRAM applications. Such use of the NOR strings allows asubstantially lower cost-per-bit figure of merit, as compared to theconventional DRAMs, and a substantially lower read-latency, as comparedto conventional NAND string arrays.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a conceptualized memory structure 100 which illustrates anorganization of memory cells according to one embodiment of the presentinvention.

FIG. 1b shows a basic circuit representation of two stacks of NORstrings sharing common vertical word-lines, according to one embodimentof the present invention.

FIG. 1c shows a basic circuit representation of a stack of four NORstrings in a Y-Z plane cross section of conceptualized memory structure100.

FIG. 2a shows a cross section in an Y-Z plane of memory structure 200,after active layers 202-0 to 202-7 have been formed on semiconductorsubstrate 201, but prior to formation of individual active strips, inaccordance with one embodiment of the present invention.

FIG. 2b -1 shows structure 220 a, which may be used to implement any ofactive layers 202-0 to 202-7 of FIG. 2a , in accordance with oneembodiment of the present invention.

FIG. 2b -2 shows structure 220 b, which includes additional metallicsublayer 224 adjacent one of layers 221 and 223 of structure 220 a, inaccordance with one embodiment of the present invention.

FIG. 2b -3 shows structure 220 c, which includes additional metallicsublayer 224 adjacent each of layers 221 and 223 of structure 220 a, inaccordance with one embodiment of the present invention.

FIG. 2c shows a cross section in an Y-Z plane through buried contacts205-0 and 205-1, which connect N⁺ sublayer 223 in each of active layers202-0 and 202-1 to contacts 206-0 and 206-1 in semiconductor substrate201.

FIG. 2d illustrates forming trenches 230 in memory structure 200 of FIG.2a , in a cross section in an X-Y plane through active layer 202-7 inone portion of memory structure 200 of FIG. 2 a.

FIG. 2e illustrates depositing charge-trapping layers 231L and 231R onopposite side walls of the active strips along trenches 230, in a crosssection in an X-Y plane through active layer 202-7 in one portion ofmemory structure 200 of FIG. 2 a.

FIG. 2f illustrates depositing polysilicon or metal 208 to fill trenches230.

FIG. 2g shows, after photo-lithographical patterning and etching stepson the memory structure of FIG. 2f , local word-lines 208 w andpre-charge word-lines 208-chg are achieved by removing exposed portionsof the deposited polysilicon 208, and filling the resulting shafts withinsulation material 209 or air gap isolation.

FIG. 2h shows a cross section in the X-Z plane through a row of localword-lines 208 w of FIG. 2g , showing active strips in active layers202-7 and 202-6.

FIG. 2i shows that each of local word-lines 208 w of FIG. 2h isconnected to either one of global word-lines 208 g-a, routed in one ormore layers provided above active layers 202-0 to 202-7, or one ofglobal word-lines 208 g-s, routed in one or more layers provided belowthe active layers between active layer 202-0 and substrate 201 (see FIG.4a ).

FIG. 2j shows an alternative embodiment to the embodiment of FIG. 2i ,in which only top global word-lines—i.e., without any bottom globalword-lines—are provided, in accordance with one embodiment of thepresent invention; in this embodiment, the local word-lines along oneedge of an active strip are staggered with respect to the localword-lines on the other edge of the active strip (see FIG. 4b )

FIG. 2k shows each of local word-lines 208 w controlling TFTs formed outof the active strips on opposite sides of the local word-line, accordingto one embodiment of the present invention (see FIG. 4c ).

FIG. 3 illustrates the methods and circuit elements used for setting asource voltage (V_(ss)) on source lines in N+ sublayer 221;specifically, the source line voltage may be set through hard-wiredecoded source line connection 280 or using pre-charge TFTs 303 andbit-line connections 270.

FIG. 4a shows a cross section in the X-Y plane, showing contacts 291connecting local word-lines 208 w to global word-lines 208 g-a, for theembodiment of the present invention shown in FIG. 2 i.

FIG. 4b shows a cross section in the X-Y plane, showing contacts 291connecting local word-lines 208 w to top global word-lines 208 g-a (orbottom global word-lines 208 g-s) in a staggered configuration, for theembodiment of the present invention shown in FIG. 2 j.

FIG. 4c shows a cross section in the X-Y plane, showing contacts 291connecting local word-lines 208 w to global word-lines 208 g-a and theisolation 209 between adjacent active strip-pairs, for the embodiment ofthe present invention shown in FIG. 2 k.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows a conceptualized memory structure 100 that facilitatesillustration in this detailed description of an organization of memorycells according to one embodiment of the present invention. As shown inFIG. 1a , memory structure 100 represents a 3-dimensional block ofmemory cells formed in deposited thin-films over the surface ofsubstrate layer 101. Substrate layer 101 may be, for example, aconventional silicon wafer used for fabricating integrated circuits,familiar to those of ordinary skill in the art. In this detaileddescription, a Cartesian coordinate system (such as indicated in FIG. 1a) is adopted solely for the purpose of facilitating discussion. Underthis coordinate system, the surface of substrate layer 101 is considereda plane which is parallel to the X-Y plane. Thus, as used in thisdescription, the term “horizontal” refers to any direction parallel tothe X-Y plane, while “vertical” refers to the Z-direction.

In FIG. 1a , each vertical column represents storage elements (i.e.,thin-film storage transistors or TFTs) that share a vertical commoncontrol gate or word-line in a stack of horizontal NOR strings, witheach NOR string running along the Y direction. Each NOR string is formedout of TFTs along an “active strip”, described in further detail below.Unlike a NAND string, in a NOR string, writing, reading or erasing oneof its TFTs does not involve activation of other TFTs in the NOR string.As shown in FIG. 1a , memory structure 100 represents an arrayconsisting of 4 stacks of NOR strings, with each stack having four NORstrings, and each NOR string having four TFTs. Note that, as aconceptualized structure, memory structure 100 is merely an abstractionof certain salient characteristics of a memory structure of the presentinvention. Although shown in FIG. 1a as an array of 4×4 NOR strings,each having four TFTs, a memory structure of the present invention mayhave any number of TFTs along any of the X, Y and Z directions. Forexample, there may be 2, 4, 8, 16, 32, 64, . . . NOR-type strings alongeach of the X and Z directions, with each NOR string may have 2, 4, 8,16, . . . 8192 or more TFTs. The use of numbers that are integer powersof 2 (i.e., 2^(n), where n is an integer) follows a customary practicein conventional memory design. It is customary to access eachaddressable unit of memory by decoding a binary address. Thus, forexample, a memory structure within the scope of the present inventionmay have M NOR strings along each of the X and Z directions, with Mbeing a number that is not necessarily 2^(n), for any integer n. Ifmemory block 100 has 8192 stacks of 8 NOR strings each, with each NORstring having 8192 storage elements, memory block 100 would have morethan half a billion storage elements in the form of NOR-type nonvolatileTFTs. As it is not uncommon today to store more than one bit in astorage element using a multi-level cell (MLC) technique, memory block100 can store more than one billion bits of information. A one-terabitmemory chip would have one thousand or more such blocks plus spareblocks available to substitute defective or worn-out blocks.

As a conceptualized structure, memory structure 100 is not drawn toscale in any of the X, Y, Z directions.

FIG. 1c shows a basic circuit representation of a stack of 4 NOR stringsin a Y-Z plane cross section of conceptualized memory structure 100. Asshown in FIG. 1c , each NOR string runs along the Y direction, withstorage elements connected between source line 153-m and bit lines154-m, where m is the index between 1 to 4 of the corresponding activestrip. Corresponding storage elements in the 4 NOR strings are connectedcorresponding vertical word-lines 151-n, where n is the index of theword-lines along the active strips.

FIG. 1b shows a basic circuit representation of two stacks of NORstrings sharing common vertical word-lines, according to one embodimentof the present invention. The detailed structure of this configurationis discussed and illustrated below in conjunction with FIG. 2k . Asshown in FIG. 1b , this basic circuit configuration includes NOR strings(e.g., NOR strings 150L and 150R) that are provided in adjacent columnsof memory structure 100 sharing a common word-line.

As shown in FIG. 1b , NOR strings 150L and 150R are NOR strings in twoactive strips located on opposite sides of common word-line 151 a.Storage transistors 152R-1 to 152R-4 and 152L-1 to 152L-4 are storageelements in the four active strips to the right and in the four activestrips to the left of their common vertical word-line 151 a,respectively. In this embodiment, as illustrated in greater detail belowin conjunction with FIG. 2k and FIG. 4c , a greater storage density maybe achieved by having a common local word-line to control TFTs ofadjacent active strips. For example, word-line 151n controls TFTs in theNOR strings of bit lines 153R-1, 153R-2, 153R-3 and 153R-4, and TFTs inthe NOR strings of bit lines 153L-1, 153L-2, 153L-3 and 153L-4. Asdiscussed in greater detail below, in one embodiment, the parasiticcapacitance C intrinsic to each NOR string (e.g., the parasiticcapacitance between the N+ diffusions of a string and its numerousassociated local word-lines) may be used, under some operatingconditions, to provide a virtual voltage source.

The TFTs in the NOR strings of the present invention may be programmed,program-inhibited, erased, or read using conventional programming,inhibition, erasure and read voltages. In one or more embodiments of thepresent invention, the TFTs are implemented by thin-film storagetransistors that are programmed or erased using Fowler-Nordheimtunneling or direct tunneling mechanisms. In another embodiment channelhot electron injection may be used for programming.

FIG. 2a shows a cross section in an Y-Z plane of memory structure 200,after active layers 202-0 to 202-7 are formed on semiconductor substrate201, in accordance with one embodiment of the present invention. Asshown in FIG. 2a , memory structure 200 includes active layers 202-0 to202-7. Semiconductor substrate 201 represents, for example, a P-dopedbulk silicon wafer on which support circuits for memory structure 200may be formed prior to forming the active layers. Such support circuitsmay include both analog and digital circuits. Some examples of suchsupport circuits may include shift registers, latches, sense amplifiers,reference cells, power supply lines, bias and reference voltagegenerators, inverters, Nand, Nor, Exclusive-Or and other logic gates,input/output drivers, address decoders, including bit-line and word-linedecoders, other memory elements, sequencers and state machines. Thesesupport circuits may be formed out of the building blocks ofconventional devices, e.g., N-Wells, P-Wells, triple wells, N⁺, P⁺diffusions, isolation regions, low and high voltage transistors,capacitors, resistors, and interconnects, as known to those skilled inthe art.

After the support circuits have been formed in and on semiconductorsubstrate 201, insulating layer 203-0 is provided, which may be adeposited or grown thick silicon oxide, for example.

Next, in some embodiments, one or more layers of interconnect may beformed, including “global word-lines,” which are discussed below. Suchmetallic interconnect lines (e.g., global word-line landing pads 264 ofFIG. 2c , discussed below) may be provided as horizontal long narrowstrips running along a predetermined direction that is perpendicular tothe active NOR strings to be formed at a later step. To facilitatediscussion in this detailed description, the global word-lines arepresumed to run along the X direction. The metallic interconnect linesmay be formed by applying photo-lithographical patterning and etchingsteps on one or more deposited metal layers. (Alternatively thesemetallic interconnect lines can be formed using a conventional damasceneprocess, such as a copper damascene process). Thick oxide 203-0 is thendeposited, followed by a planarization step using conventional chemicalmechanical polishing (CMP).

Active layers 202-0 to 202-7 are then successively formed, each activelayer being insulated from previous active layer underneath by acorresponding one of insulating layers 203-1 to 203-7. In FIG. 2a ,although eight active layers are shown, any number of active layers maybe provided. In practice, the number of active layers to provide maydepend on the process technology, such as availability of awell-controlled anisotropic etching process that allows cutting throughthe active layers to reach semiconductor substrate 201. Each activelayer is etched at an etching step discussed below to form a largenumber of parallel active strips each running along the Y direction.

FIG. 2b -1 shows structure 220 a, which may be used to implement any ofactive layers 202-0 to 202-7 of FIG. 2a , in accordance with oneembodiment of the present invention. As shown in FIG. 2b -1, activelayer 220 a includes deposited polysilicon sublayers 221-223. Sublayers221-223 may be deposited successively in the same process chamberwithout removal in between. Sublayer 223 may be formed by depositing5-50 nm of in-situ doped N⁺ polysilicon. Sublayers 222 and 221 may thenbe formed by depositing undoped or lightly doped polysilicon, in thethickness range of 40-100 nm. Sublayer 221 (i.e., the top portion of thedeposited polysilicon) is then N⁺ doped. This N⁺ doping may be achievedby either (i) a low-energy shallow ion implantation of arsenic orantimony, forming a 20-50 nm N+ doped top sublayer 221, or (ii) in-situdoping of the deposited polysilicon, forming a 20-50 nm N⁺ top sublayer221. (Thermal diffusion should not be used, as it would expose the loweractive layers formed earlier to greater diffusion than the upper activelayers). A low-dose implantation of boron (P−) or phosphorus (N−) ionsmay also be carried out at an energy sufficient to penetrate theimplanted or in-situ N⁺ doped sublayer 221, so as to adjust to anenhancement mode threshold voltage for sublayer 222 lying between top N+doped sublayer 221 and bottom N+ doped sublayer 223.

Thermal activation of the N+ and P− implanted species in sublayers 221and 222 should preferably take place after all active layers 202-0 to202-7 have been formed, using a conventional rapid thermal annealingtechnique (e.g., at 700° C. or higher), thereby ensuring that all activelayers experience high temperature processing in roughly the sameamount. Caution must be exercised to limit the total thermal budget, soas to avoid merging N⁺ sublayer 223 with the N+ sublayer 221, so as notto eliminate P⁻ sublayer 222. P⁻ sublayer 222 is required to remainsufficiently thick to avoid N+P−N+ transistor punch-through at a lowvoltage applied across N+ sublayers 221 and 223.

The final thickness of sublayer 222 represents the TFT channel length,which may be as little as 10 nm or less over long active strips. In oneembodiment, it is possible to control the TFT channel length to lessthan 10 nm by depositing an ultra-thin (around 1 nm) film of siliconnitride (e.g., SiN or Si₃N₄), or another suitable diffusion-blockingfilm following the formation of N⁺ sublayer 223, and then againfollowing deposition of the polysilicon of sublayer 222 in a thicknessrange between 5-30 nm, before depositing N+ polysilicon sublayer 221.The ultra-thin silicon nitride layers can be deposited by chemical vapordeposition, atomic layer deposition or any other means, e.g., highpressure nitridization at low temperature. Each ultra-thin siliconnitride layer acts as a diffusion-barrier that prevents the N⁺ dopantsin N⁺ sublayers 221 and 223 from diffusing into P⁻ sublayer 222, yet aresufficiently thin to only marginally impede MOS transistor action in theregion between N+ sublayer 221 (acting as a source) and N+ sublayer 223(acting as a drain). (Electrons in the surface inversion layer ofsublayer 222 readily tunnel directly through 1 nm of silicon nitride).These additional ultra-thin silicon nitride layers increase themanufacturing cost, but serve to significantly reduce leakage current inthe numerous TFTs along the active strips that are in the “off” state,while providing a high read current for the accessed TFT that is in the“on” state.

Optionally, to provide lower resistivity along the bit lines and thesource lines of N⁺ sublayers 223 and 221, additional conductive sublayer224 may be provided adjacent the corresponding one of N⁺ sublayers 221and 223 (e.g., w in FIG. 2b -2), or both (e.g., FIG. 2b -3). Sublayer224 may be provided by one or more deposited metal layers. For example,sublayer 224 may be provided by first depositing 1-2 nm thick layer ofTiN, followed by depositing a 10-40 nm thick layer of tungsten or asimilar refractory metal, or its silicide or salicide. Reduced lineresistance is desirable for reducing the “RC delay” of a signaltraversing a long conductive strip (i.e., the time delay due to theproduct of the line resistance R and the line capacitance C), and forminimizing the “IR drop” across a long and narrow active strip (i.e.,the voltage drop due to the product of the current I and the lineresistance R). Inclusion of metal sublayer 224 in each of active layers202-0 to 202-7 may, however, increase cost and complexity in themanufacturing process, including the complication that some of themetallic materials are relatively more difficult to anisotropically etchthan materials such as polysilicon or silicon oxide in the othersublayers. However, the use of metal sublayer 224 enables use ofconsiderably longer active strips which result in superior arrayefficiency. On the other hand, shorter active strips have superiorimmunity to leakage between N⁺ sublayer 223 and N⁺ sublayer 221, andlower intrinsic capacitance than the longer strips. The integratedcircuit designer may opt for a shorter active strip (with or without oneor both metal layers) when low latency is most valued. Alternatively thestrip resistance may be reduced by providing buried contacts at bothends, of each active strip, rather than just at one end.

Block-formation patterning and etching steps define separate blocks inthe active layers formed. Each block defines an area in which a largenumber (e.g., thousands) of active strips running in parallel along theY direction may be formed, as discussed below, with each active stripeventually forming a large number (e.g., thousands) of TFTs.

Each of active layers 202-0 to 202-7 is successively formed, with eachactive layer being formed by repeating the steps described above. Inaddition, in the block-formation patterning that defines the blocks ofeach active layer, each next higher active layer extends slightly beyondthe previous active layer (see, e.g., as illustrated in FIG. 2c ,discussed below, layer 202-1 extends beyond layer 202-0) to allow thehigher active layer to access its specific decoders and other circuitryin semiconductor substrate 201 through designated buried contacts.

FIG. 2c shows a cross section in an Y-Z plane through buried contacts205-0 and 205-1, which connect N⁺ sublayer 223 in each of active layers202-0 and 202-1 to contacts 206-0 and 206-1 in semiconductor substrate201. As shown in FIG. 2c , buried contacts 205-0 and 205-1 connectcontacts 206-0 and 206-1 in semiconductor substrate 201, for example, tothe local bit or source lines formed out of N⁺ sublayer 223 in each ofactive layers 202-0 and 202-1. Buried contacts for active layers 202-2to 202-7 (not shown) may be similarly provided to connect active layers202-2 to 202-7 to contacts 206-2 to 206-7 (not shown) in semiconductorsubstrate 201. Through a switch circuit, each of contacts 206-0 to 206-7may apply a pre-charge voltage V_(b1) to the respective bit line orsource line or, during a read operation, may be connected to an inputterminal of a sense amplifier or a latch. The switch circuit mayselectively connect each of contacts 206-0 to 206-7 to any of a numberof specific voltage sources, such as a programming voltage(V_(program)), an inhibit voltage (V_(inhibit)), an erasure voltage(V_(erase)), or any other suitable predetermined or pre-charge referencevoltage V_(b1) or V_(ss). In one embodiment, discussed below, using therelatively large parasitic capacitance C along a bit line or sourceline, a virtual ground may be created in sublayer 221 of each activelayer. In that embodiment, buried contacts and separate interconnectsneed not be provided to the bit or source lines formed out of N⁺sublayer 221 in each of active layers 202-0 to 202-7.

FIG. 2c also shows buried contacts 261-0 to 261-n for connecting globalword-lines —which are to be formed running along the X direction—tocontacts 262-0 to 262-n in semiconductor substrate 201. These globalword-lines are provided to connect corresponding local word-lines 208 wto be formed (see, e.g., FIG. 2g , which is described below). Landingpads 264 are provided to allow connection to local word-lines 208 w,which are yet to be formed vertically on top of global word-lines 261-0and 261-n. Through a switch circuit and a global word-line decoder, eachof global word-lines 262-0 to 262-n may be selectively connected, eitherindividually, or shared among several global word-lines to any one of anumber of reference voltage sources, such as stepped programming voltage(V_(program)), a read voltage (V_(read)) and an erasure voltage(V_(erase)).

These buried contacts, the global word-lines and the landing pads may beformed using conventional photo-lithographical patterning and etchingsteps, following by deposition of one or more conductors or by alloying(e.g., tungsten metal or tungsten silicide).

After the top active layer (e.g., active layer 202-7) is formed,trenches are created by etching through the active layers to reach thebottom global word-lines (or semiconductor substrate 201) using astrip-formation mask. The strip-formation mask consists of a pattern ina photoresist layer of long narrow strips running along the Y direction(i.e., perpendicular to that of global word-line strips that run alongthe X direction). Sequential anisotropic etches etch through activelayers 202-7 down to 202-0, and dielectric isolations layers 203-7 downto 203-0. As the number of active layers to be etched, which is eight inthe example of FIG. 2c (and, more generally may be 16 or more activelayers), a photoresist mask by itself may not be sufficiently robust tohold the strip pattern through the numerous etches necessary to etchthrough the lowest active layer. Thus, reinforcement by a hard maskmaterial, such as carbon, may be required, as is known to those ofordinary skill in the art. Etching terminates at the dielectricisolation above the landing pads of the global word-lines. It may beadvantageous to provide an etch-stop barrier film such as aluminum oxideto protect the landing pads during the trench etch sequence.

FIG. 2d illustrates forming trenches 230 in memory structure 200 of FIG.2a , in a cross section in an X-Y plane through active layer 202-7 inone portion of memory structure 200 of FIG. 2a . Between adjacenttrenches 230 is a stack of high aspect-ratio, long and narrow activestrips. To achieve the best etch result, etch chemistry may have to bechanged when etching through the materials of the different sublayers,especially when metal sublayers 224 are present. The anisotropy of themulti-step etch is important, as undercutting of any sublayer should beavoided as much as possible, and so that an active strip in the bottomactive layer (e.g., an active strip in active layer 202-0) hasapproximately the same width and gap spacing to an adjacent active stripas the corresponding width and gap spacing in an active strip of the topactive layer (i.e., an active strip of active layer 202-7). Naturally,the greater the number of active layers in the stack to be etched, themore challenging is the design of the successive etches. To alleviatethe difficulty associated with etching through, for example, 32 activelayers, etching may be conducted in sections of, say 8 layers each, asdiscussed in Kim, mentioned above, at pp. 188-189. As shown in FIG. 2d ,trenches 230 run along the Y direction.

Thereafter, one or more layers of charge-trapping material areconformally deposited on the sidewalls of the active strips in trenches230. The charge-trapping layer is formed by first depositing or growinga thin tunneling dielectric film of a 2-10 nm thickness, typically asilicon dioxide layer or a silicon oxide-silicon nitride-silicon oxide(“ONO”) triple layer, followed by deposition of a 4-10 nm thick layer ofcharge-trapping material, typically silicon nitride or silicon-richnitride or oxide or nanocrystals or nanodots embedded in a thindielectric film, which is then capped by a blocking dielectric. Theblocking dielectric may be a 5-15 nm thick layer consisting, forexample, of an ONO layer, or a high dielectric constant film, such asaluminum oxide, hafnium oxide or some combination thereof. The storageelement can be SONOS, TANOS, nanodot storage, isolated floating gates orany suitable charge-trapping sandwich structures known to those ofordinary skill in the art. Trenches 230 must be sufficiently wide toaccommodate the storage elements on the two opposing sidewalls of theadjoining active strips, plus the vertical local word-lines to be sharedby the TFT's on these opposite sidewalls. FIG. 2e illustrates depositedcharge-trapping layers 231L and 231R on opposite side walls of theactive strips along trenches 230, in a cross section in an X-Y planethrough active layer 202-7 in one portion of memory structure 200 ofFIG. 2 a.

Contact openings at the bottom global word-lines arephoto-lithographically patterned at the top of layer 202-7 and exposedby anisotropically etching through the charge-trapping materials at thebottom of trenches 230, stopping at the bottom global word-line landingpads (e.g., global word-line landing pads 264 of FIG. 2c ). In oneembodiment, to be described in conjunction with FIG. 2i below, onlyalternate rows of trenches 230 (e.g., the rows in which the word-linesformed therein are assigned odd-numbered addresses) should be etcheddown to the bottom global word-lines. In some embodiments, etching ispreceded by a deposition of an ultra-thin film of polysilicon (e.g. 2-5nm thick) to protect the vertical surface of the blocking dielectric onthe sidewalls of trenches 230 during the anisotropic etch of thecharge-trapping material at the bottom of trenches 230.

Thereafter, doped polysilicon (e.g., P⁺ polysilicon) may be depositedover the charge-trapping layers to form the control gates or verticallocal word-lines. P⁺ doped polysilicon is preferable because of itshigher work function than N+ doped polysilicon. Alternatively, a metalwith a high work function relative to SiO₂ (e.g., tungsten, tantalum,chrome or nickel) may also be used to form the vertical local wordlines. Trenches 230 may now be filled with the P⁺ doped polysilicon orthe metal. In the embodiment of FIG. 2i , discussed below, the dopedpolysilicon or metal in alternate rows of trenches 230 (i.e., the rowsto host local word-lines that are assigned odd-numbered addresses) is inohmic contact with the bottom global word-lines. The polysilicon in theother ones of trenches 230 (i.e., the rows to host local word-lines thatare assigned even-numbered addresses) are isolated from the bottomglobal word-lines. (These local word-lines are to be contacted by topglobal word-lines routed above the top active layer). The photoresistand hard mask may now be removed. A CMP step may then be used to removethe doped polysilicon from the top surface of each block. FIG. 2fillustrates depositing polysilicon 208 to fill trenches 230.

FIG. 2g shows, after photo-lithographical patterning and etching stepson the memory structure of FIG. 2f , local word-lines 208 w are achievedby removing exposed portions of the deposited polysilicon 208, andfilling the resulting shafts with insulation material 209. As theremoval of the doped polysilicon in this instance is a high aspect-ratioetch in a rather confined space, a hard mask may be required, using thetechnique described above. The resulting shafts may be filled withinsulation material 209 or left as an air gap. The mask pattern thatexposes the doped polysilicon for excavation are parallel strips thatrun along the X direction, so that they coincide with the globalword-lines that are required to be formed in one embodiment to contactlocal word-lines 208 w.

In FIG. 2g , the portions of charge-trapping layers 231L and 231Radjacent insulation material 209 remained after the removal of thecorresponding portions of deposited polysilicon 208. In someembodiments, those portions of charge-trapping layers 231L and 231R maybe removed by a conventional etching process prior to filling the shaftswith insulation material 209. Etching of the charge-trapping materialsin the shafts may be carried out concurrently with the removal of thedoped polysilicon, or subsequent to it. A subsequent etch would alsoremove any fine polysilicon stringers that the anisotropic etch has leftbehind; such polysilicon stringers may cause undesirable charge leakage,serving as resistive leakage paths between adjacent vertical localword-lines. Removal of such charge trapping material also eliminateslateral diffusion of trapped charge between one TFT and the TFTsimmediately to its left and right along the same string.

FIG. 2h shows a cross section in the X-Z plane through a row of localvertical word-lines 208 w (shown also in FIG. 2g in the X-Y plane),showing active strips in active layers 202-7 and 202-6. As shown in FIG.2h , each active layer includes N⁺ sublayer 221, P⁻ sublayer 222, and N⁺sublayer 223. In one embodiment, N+ sublayer 221 (e.g., a source line)is connected to a ground reference voltage V_(ss) (not shown) and N⁺sublayer 223 (e.g., a bit line) is connected to a contact in substrate201 according to the method illustrated in FIG. 2c . Thus, localword-line 208 w, the portion of active layer 202-7 or 202-6 facingword-line 208 w and the charge-trapping layer 231L between word-line 208w and that portion of active layer 202-7 or 202-6 form a storage elementor storage TFT, as indicated by reference numeral 281 and 282 in FIG. 2h. Facing TFTs 281 and 282 on the opposite side of 208W are TFTs 283 and284 respectively, incorporating therein charge trapping layer 231R. Onthe other side of the active strips 202-6 and 202-7 providing TFTs 283and 284 are TFTs 285 and 286. Accordingly, the configuration shown inFIG. 2h represents the highest packing density configuration for TFTs,with each vertical word-line shared by the two active strips along itssides, with each active strip being shared by the two word-lines alongits sides. N⁺ sublayer 223 can be charged to a suitable voltage requiredfor an operation of the storage transistors at hand (e.g., programvoltage V_(prog), inhibition voltage V_(inhibit), erase voltageV_(erase), or the read reference voltage V_(b1)). As shown in FIG. 2h ,additional metallic sublayer 224 increases the conductivity of the bitline, so as to facilitate memory device operations. In anotherembodiment, N⁺ sublayer 221 in any of active layers 202-0 to 202-7 maybe left floating. In each active layer, one or more of the localvertical word-lines (referred to as a “pre-charge word-line”; e.g.,pre-charge word-lines 208-chg in FIG. 2g ) may be used as a non- memoryTFT. When a suitable voltage is applied (i.e., rendering the pre-chargeTFT to the “on” state), each pre-charge word-line momentarily invertssublayer 222, so that N⁺ sublayer 221 may be pre-charged to the voltageV_(ss) on N⁺ sublayer 223. When the voltage on the pre-charge word-lineis withdrawn, (i.e., returned to its “off ” condition) and all the otherword-lines on both sides of the strip are also “off”, device operationmay proceed with N⁺ sublayer 221 left electrically charged as a virtualreference at the pre-charged voltage V_(ss) because the parasiticcapacitance in the strip capacitor of N⁺ sublayer 221 is large enough tohold its charge sufficiently long to support program and read operations(see below).

Each local word-line 208 w may be used to read, write or erase thecharge stored in the designated one of the TFTs formed in each of activelayers 202-0 to 202-7, located on either charge-trapping portion 231L or231R, when a suitable voltage is imposed. Alternatively, in oneembodiment, to be described in conjunction with FIG. 2k below, eachlocal word-line 208 w may be used to read, write or erase the chargestored in any of the TFTs formed in each of active layers 202-0 to202-7, located on either charge-trapping portions 231L or 231R, when asuitable voltage is imposed. However, as shown in FIG. 2k , only one ofthe two sides of active layers 202-0 to 202-7 is formed as storage TFTs,thereby eliminating the need for both bottom and top global word-linesin the embodiment.

An isolation dielectric or oxide may be then deposited and its surfaceplanarized. Contacts to semiconductor substrate 201 and to localword-lines 208 w may then be photo-lithographically patterned andetched. In one embodiment, which is described in conjunction with FIG.2i and corresponding FIG. 4a , contacts to local word-lines 208 w areprovided only for those assigned an even-numbered address (localword-lines assigned an odd address are contacted from the bottom of thearray by bottom global word-lines). For the embodiment shown in FIG. 2j, contacts are provided for every local word-line but the localword-lines are staggered relative to opposing word-lines, as shown inFIG. 4b . A deposited metal layer provides a top metal layer and thecontacts. Such a metal layer may be provided by, first, forming a thinTiN layer, followed by forming a low resistance metal layer (e.g.,tungsten). The metal layer is then photo-lithographically patterned toform top global word-lines. (Alternatively, these global word-lines maybe provided by a copper damascene process.) In one embodiment, theseglobal word-lines are horizontal, running along the X direction,electrically connecting with the contacts formed in the isolation oxide(i.e., thereby contacting local word-lines 208 w) and with the contactsto semiconductor substrate 201 (not shown). Of course other mask andetch process flows known to those skilled in the art are possible toform even and odd addressed local word-lines and connect themappropriately to their global word-lines, either from the top of thearray through the top global word-lines or from the bottom of the arraythrough the bottom global word-lines and in some embodiments from both.

FIG. 2i shows that each of local word-lines 208 w of FIG. 2h isconnected to either one of global word-lines 208 g-a, routed in one ormore layers provided above active layers 202-0 to 202-7, or one ofglobal word-lines 208 g-s, routed in one or more layers provided belowthe active layers between active layer 202-0 and substrate 201. Localword-lines 208 w that are coupled to bottom global word-lines may beassigned odd addresses, while local word-lines 208 w coupled to the topglobal word-lines may be assigned even addresses. FIG. 4a shows acorresponding cross section in the X-Y plane, showing contacts 291connecting local word-lines 208 w to global word-lines 208 g-a. (Incontrast, in the embodiment of FIG. 2k and corresponding FIG. 4c , localword-lines 208 w control each active strip on only one of the activestrip's sides.)

FIG. 2j shows an alternative embodiment to the embodiment of FIG. 2i ,in which only top global word-lines are provided (or, alternatively,only bottom global word-lines are provided), in accordance with oneembodiment of the present invention. In this embodiment, the localword-lines along one edge of an active strip are staggered with respectto the local word-lines on the other edge of the active strip. This isshown in FIG. 4b , which shows a corresponding cross section in the X-Yplane, showing contacts 291 connecting local word-lines 208 w to topglobal word-lines 208 g-a (or bottom global word-lines 208 g-s) in astaggered configuration. This embodiment simplifies the process flow byobviating the process steps needed to form the bottom global word-lines(or the top global word-lines, as the case may be). In the embodiment ofFIG. 2i and corresponding FIG. 4a , in which both top and bottom globalword-lines are provided, two TFTs may be provided in each active layerof each active strip within one pitch of a global word-line (i.e., ineach active strip, one TFT is formed using one sidewall of the activestrip, and controlled from a bottom global word-line, the other TFT isformed using the other sidewall of the active strip, and controlled froma top global word-line). (A pitch is one minimum line width plus arequired minimum spacing between adjacent lines). In contrast, as shownin FIG. 2j and corresponding FIG. 4b , only one TFT may be providedwithin one global word line pitch in each active layer. The two localword-lines 208 w at the two sides of each strip can be staggeredrelative to each other so that two global word-line pitches are requiredto contact them both. The penalty for the staggered embodiment is theforfeiting of the double-density TFT inherent in having both edges ofeach active strip providing a TFT within one pitch of each globalword-line.

FIG. 2k shows each of local word-lines 208 w controlling TFTs formed outof the active strips on opposite sides of the local word-line, accordingto one embodiment of the present invention. FIG. 4c shows acorresponding cross section in the X-Y plane, showing contacts 291connecting local word-lines 208 w to global word-lines 208 g-a and theisolation 209 between adjacent active strip-pairs. As shown in FIG. 2k ,each TFT is formed from either one of a dual-pair of active stripslocated on opposite side of a common local word-line, with eachdual-pair of active strips being isolated by a trench filled with oxideor dielectric material or an air gap 209 from similarly formed adjacentdual-pairs of active strips. The isolation trenches between adjacentdual-pairs of active strips accommodate the charge-trapping-material 231or polysilicon 208. After the local word-lines 208 w are defined byetching, the protected isolation trenches may then be filled with oxideor dielectric material 209, or left as air gaps.

FIG. 3 shows schematically an embodiment where sublayer 221 of eachactive strip (see, e.g., FIG. 2b -1) is connected by hard wires 280(dashed line) to a source reference voltage V_(ss) by a metal or N+doped polysilicon conductor. Each of hardwires 280 may be independentlyconnected, so that the source voltages for different layers need not bethe same. Since sublayer 221 is formed only after sublayer 223 isformed, the metal or N+ doped polysilicon conductor to connect sublayer221 to the reference voltage V_(ss) requires one or two additionalpatterning and etching steps to each of active layers 202-0 to 202-7,hence increasing the processing cost. To avoid this added cost, use ismade of the large intrinsic parasitic capacitance C of each active NORstring. Taking advantage of intrinsic parasitic capacitance C, hardwires 280 are not needed and sublayer 221 of each active strip is leftfloating, after being temporarily pre-charged to voltage V_(ss)transferred to it from bit-line sublayer 223 through the action of localvertical pre-charge TFTs that are controlled by pre-charge word-lines208-chg. In a long horizontal NOR string (e.g., with 1,024 or morememory TFTs), several pre-charge TFTs may be provided on either side ofthe active strip (e.g., one every 512 TFTs). Assuming a local capacitorbetween each local word-line as one plate and N+/P−/N+ active layer asthe other plate, each such TFT provides a capacitor that is typicallyaround 3×10¹⁸ farads. As there are slightly more than 2,000 TFTscontributing capacitance from both sides of the strip, the totalcapacitance C of the string is approaching 0.01 picofarad, which issufficient to preserve the pre-charge voltage on it well beyond themilliseconds required for write, erase or read operations that followimmediately after a pre-charging operation. Capacitance C may beincreased by lengthening the NOR string to accommodate thousands moreTFTs along each side of the string, correspondingly increasing theretention time of pre-charge voltage V_(ss) on N⁺ sublayer 221. However,a longer NOR string suffers from an increased leakage current between N⁺sublayer 221 and N⁺ sublayer 223, such leakage current may interferewith the sensed current when reading the one TFT being addressed. Also,the potentially longer time it takes to pre-charge a larger capacitorduring a read operation can conflict with the requirement for a low readlatency (i.e., a fast read access time). To speed up the pre-charging ofthe capacitance C of a long NOR string would typically require providingmore than one pre-charge TFT; such pre-charge TFTs may be distributedthroughout the length of the NOR string.

Also shown in FIG. 3 is an optional connection 290 to the P− sublayers222 to access a back-bias voltage V_(bb) from substrate 201. A negativeV_(bb) voltage can be used to modulate the threshold voltage of the TFTsalong each active strip, so as to reduce the subthreshold leakagecurrent between N+ source sublayer 221 and N+ drain sublayer 223. Insome embodiments a high positive V_(bb) voltage can be applied during anerase operation to tunnel-erase TFTs whose control gates are held atground potential.

Because the TFTs in a NOR string are connected in parallel, the readoperating condition for the NOR strings of the present invention shouldpreferably ensure that all TFTs along both edges of an active stripoperate in enhancement mode (i.e., they each have a positive thresholdvoltage between their control gate 151n and their source 221 voltageV_(ss)), so as to suppress the leakage current between N⁺ sublayers 221and 223 of the active strip when all control gates on both sides of thestrip are held at, or below V_(ss). This enhancement threshold voltagecan be achieved by doping sublayer 222 with a P− dopant concentration(typically by boron, in a concentration typically between 1×10¹⁶ and1×10¹⁷ per cm³), so as to result in a native TFT threshold voltage ofaround 1 volt, and by holding all unaddressed local word-lines on bothsides of the active strip at 0 volt. Alternatively, if some of the TFTsalong an active string have negative threshold voltage (i.e in depletionmode threshold voltage), leakage current suppression can be achieved byraising the V_(ss) voltage on N⁺ sublayer 221 to around 1.5 volts andthe voltage V_(b1) on N⁺ sublayer 223 to a voltage that is about half avolt to one volt above 1.5 volts, while holding all local word-lines at0 volt. This provides the same effect as holding the word-line voltageat −1.5 volts with respect to the source, thereby suppressing anyleakage due to TFTs that are in a slightly depleted threshold voltage.Also, after erasing a NOR string, the erase operation should preferablyinclude a soft-programming operation that shifts any TFT that has beenover-erased into a depletion mode threshold voltage back into anenhancement mode threshold voltage.

The charge-trapping material (e.g., an ONO stack) described above has along data retention time (typically measured in many years), but lowendurance. Endurance, which is a measure of a storage transistor'sperformance degradation after some number of write-erase cycles, istypically considered low if it is less than ten thousand cycles.However, one may alter the charge-trapping material to reduce retentiontimes, but significantly increase endurance (e.g., reducing retentiontimes to hours, while increasing endurance to tens of millions ofwrite/erase cycles). For example, in an ONO film or a similarcombination of charge-trapping layers, the tunnel dielectric, typically6-8 nm of silicon oxide can be reduced to 2 nm or less of silicon oxide,or replaced altogether with another dielectric (e.g., silicon nitride orSiN). Under a modest positive control gate voltage, electrons will beattracted through direct tunneling (as distinct from Fowler-Nordheimtunneling) into the silicon nitride charge-trapping layer where theelectrons will be temporarily trapped for a few minutes to hours ordays. The charge-trapping silicon nitride layer and the blocking layerof silicon oxide or aluminum oxide will keep these electrons fromescaping to the control gate word-line, but they will eventually leakback out to the active sublayers (electrons are negatively charged andrepel each other). Even if the 2 nm or less tunnel dielectric breaksdown locally after extended cycling, the trapped electrons will be slowto depart from their traps in the silicon nitride layer. Othercombinations of charge storage materials may also result in a highendurance but low retention (“semi-volatile”) TFT. Such a TFT mayrequire periodic write refresh to replenish the lost charge. Becausesuch a TFT provides a relatively fast read access time with a lowlatency, NOR string arrays of the present invention that have such TFTsmay be useful in applications that currently can get by with relativelyslow DRAMs. The advantages of such NOR string arrays over DRAMs include:a much lower cost-per-bit because DRAMs cannot be built in threedimensional blocks, and a much lower power dissipation, as the refreshcycles need only be run approximately once every few minutes or onceevery few hour, as compared to every few milliseconds required incurrent DRAM technology. The NOR string arrays of the present inventionare achieved by changing the constitution of the charge-trappingmaterial (e.g., charge-trapping layers 231L and 231R in FIG. 2e ), andby appropriately adapting the program/read/erase conditions toincorporate the periodic data refreshes.

According to another embodiment of the present invention, NOR stringarrays may also be programmed using a channel hot-electron injectionapproach, similar to that which is used in NROM/Mirror Bit transistors,known to those of ordinary skill in the art. In an NROM/Mirror Bittransistor, charge representing one bit is stored at one end of thechannel region next to the junction with the drain region, and byreversing polarity of the source and drain, charge representing a secondbit is programmed and stored at the opposite end of the channel regionnext to the source junction. Typical programming voltages are 5 volts onthe drain, 0 volt on the source and 8 volts on the control gate. Readingboth bits requires reading in reverse the source and drain, as is wellknown to those skilled in the art. However, channel hot electronprogramming is much less efficient than tunnel programming, andtherefore the method does not lend itself to the massively parallelprogramming possible by tunneling. The channel hot electron injectionapproach provides double the bit density though, making it attractivefor applications such as archival memory.

Exemplary operations for the NOR strings of the present invention arenext described.

Read Operation

To read a TFT among the many TFTs on an active strip, the TFTs on bothsides of the active strip are initially set in an “off” state, so thatall global and local word-lines in a selected block are initially heldat 0 volts. In FIG. 3, the addressed NOR string can either share asensing circuit among several NOR strings through a decoding circuitry,or each NOR string may be directly connected to a dedicated sensingcircuit and many other addressed NOR strings sharing the same plane canbe sensed in parallel. Each addressed NOR string has its source (N⁺sublayer 221) set at V_(ss)˜0V, either through hard-wire 280 or throughbit-line connection 270 in conjunction with pre-charge word-line 208-chg(in this case V_(b1) is initially held at 0 volt during a pre-chargephase). After the pre-charge phase, the bit-line (i.e., N⁺ sublayer 223)is then set to around V_(b1)˜2 volts through bit-line connection 270.The V_(b1) voltage is the sense voltage at the sense amplifiers for theaddressed NOR strings. The one addressed global word-line and all itsassociated vertical local word-lines are raised from 0 volts totypically around 2 volts while all other global word lines in the blockare in their off state. If the addressed TFT is in an erased state(i.e., V_(th)˜1 volt), bit-line voltage V_(b1) will begin to dischargetoward source voltage V_(ss). This voltage dip is detected by therespective sense amplifier. However, if the addressed TFT is in aprogrammed state (e.g, V_(th)˜3 volts) no dip is detected.

When MLC is used (i.e., more than one bit of information is stored ineach TFT), the addressed TFT may be programmed to any of severalthreshold voltages (e.g., 1 volt (for an erased state), 2.5 volts, 4volts or 5.5 volts for the four states representing two bits of data).The addressed global word-line and its local word-lines can be raised inincremental voltage steps until conduction is detected in the addressedTFT by the respective sense amplifier. Alternatively a single word-linevoltage V_(b1) can be applied (e.g. V_(b1)=6 volts), and the rate ofdischarge of voltage V_(b1) can be compared with the rate of dischargeof each of several programmable reference voltages representative of thefour voltage states representing the bits stored in the TFT. Thisapproach can be extended to store eight states (for 3-bit MLC TFT) or acontinuum of states, thereby effectively providing analog storage. Theprogrammable reference voltages are stored on NOR strings dedicated asreference NOR strings in the same block, preferably located in the sameplane as the addressed NOR string. When MLC is used, more than oneprogrammable reference NOR string may be provided to detect each of theprogrammed states. For example, if 3-bit MLC is used, there should be atleast seven reference NOR strings; preferably, an entire set ofreference NOR strings should be provided for each active layer and eachblock. The programmable reference NOR strings closely track thecharacteristics of the operating NOR strings in the same block throughread, program, and background leakage. Only TFTs on one of the two sidesof the active strip can participate in a read operation; each TFT on theother side of an active strip must be set to the “off” state. Other waysto read the correct state of a multi-state TFT, as are known to thoseskilled in the art.

Reading is fast because, in a NOR string, only the TFT to be read isrequired to be “on”, as compared to a NAND string, in which the TFTs inseries with the one TFT being read must also be “on”. In the embodimentsin which metal sublayer 224 is not provided in the active layer (see,e.g., 220 a of FIG. 2b -1), for a string with 1,024 TFTs on each side, atypical resistance R is ˜100,000 Ohm and a typical capacitance C˜10¹⁴farad, to provide a RC time delay in the order of one nanosecond. Evenwith 4,098 TFTs in each NOR string on either side of an active strip,the RC time delay would still be less than 20 nanoseconds. The timedelay may be much reduced, if metal sublayer 224 is provided to reducethe resistance R of the active strip. To further reduce read latency,some or all the planes in selected active blocks may be kept alwayspre-charged to their read voltages V_(ss) and V_(b1), thereby renderingthem ready to immediately sense the addressed TFT (i.e., obviating thepre-charge phase immediately before the read operation). Suchready-standby requires very little standby power because the currentrequired to periodically re-charge capacitor C to compensate for chargeleakage is very small. Within each block, all strings on all eight ormore planes can be pre-charged to be ready for fast read; for example,after reading all strings in plane 207-0 (FIG. 2a ), plane 207-1 can beread in short order because its V_(ss) and V_(b1) are already previouslyset for a read.

In memory block 100, only one TFT per NOR string can be read in a singleoperation. In a plane with eight thousand NOR strings, the eightthousand TFTs that share a common global word-line may all be readconcurrently, provided that each NOR string is connected to its ownsense amplifier. If each sense amplifier is shared among, for example,four NOR strings in the same plane using a string decode circuit, thenfour read operations are required to take place in four successivesteps, with each read operation involving two thousand TFTs. Each planecan be provided its own set of dedicated sense amplifiers or,alternatively one set of sense amplifiers can be shared among NORstrings in the eight or more planes through a plane-decoding selector.Providing separate sense amplifiers for each plane allows concurrentread operations of NOR strings of all planes, which correspondinglyimproves the read throughput. However, such throughput comes at theexpense of the extra chip area needed for the additional senseamplifiers, and may also create ground voltage bounces when too manyTFTs are read all at once. In that regard, embodiments relying onpre-charged capacitor C to set the virtual V_(ss) voltage areparticularly advantageous, as it eliminates such ground voltage bouncesbecause source voltage V_(ss) of all NOR strings is not connected to thechip's V_(ss) ground line.

Program (Write) and Program-Inhibit Operations

There are several ways to program an addressed TFT to its intendedthreshold voltage. The most common way is by tunneling, i.e., eitherdirect tunneling or Fowler-Nordheim tunneling. Either one of thesetunneling and charge-trapping mechanisms is highly efficient, so thatvery little current is needed to program a TFT, allowing parallelprogramming of tens of thousands of TFTs with minimal power dissipation.For illustration purpose, let us assume that programming by tunnelingrequires a 20 volts pulse of 100 microseconds (us) duration to beapplied to the addressed word-line (control gate), with 0 volts appliedto the active strip (e.g., 202-0 in FIG. 2a ). Under these conditions N+source and drain (sublayers 221, 223 in FIG. 2b -1) and the P− channel(sublayer 222) of the TFT is inverted at the surface and electronstunnel into the charge trapping layer. TFT Programming can be inhibitedby applying a half-select voltage (e.g., 10 volts, in this example).Program-inhibit can be accomplished, for example, either by lowering theword-line voltage to 10 volts while keeping the strip voltage at 0 volt,or by raising to 10 volts the active strip voltage, while keeping theword-line voltage at 20 volts, or some combination of the two. Only oneTFT on an addressed active strip can be programmed at one time, but TFTson other strips can be programmed in parallel in the same programmingcycle. When programming one of the many TFTs on one side of an addressedactive strip (e.g., one TFT in the even-addressed NOR string), all otherTFTs in the NOR strings are programming-inhibited, as are all TFTs onthe other side of the active strip (e.g., all TFTs in the odd-addressedNOR string). Once the addressed TFT is programmed to the targetthreshold voltage of its designated state, programming-inhibition ofthat TFT is required, as overshooting that target voltage will exertunnecessary stress on the TFT. When MLC is used, overshooting the targetvoltage may cause overstepping or merging with the threshold voltage ofthe next higher target threshold voltage state. It should be noted thatall TFTs in the adjacent active strips on the same plane sharing thesame global word-line and its associated local word-lines—thus, areexposed to the 20 volts programming voltage—and are required to beprogramming-inhibited once they have been programmed to their targetthreshold voltages. Similarly, all TFTs on other planes that are withinthe same block and that share the same global word-line and itsassociated local word-lines—and thus, are also exposed to the 20 voltsprogramming voltage—are also required to be programming-inhibited. Theseprogram and program-inhibit conditions can all be met under the presentinvention because the even and odd sides of each active strip arecontrolled by different global word-lines and their associated localword-lines, and because the voltage on each active strip regardless ofits plane can be set independently from all other active strips or otherplanes.

In one example, all TFTs in a block are first erased to a thresholdvoltage of around 1 volt. The voltage on the active strip of eachaddressed TFT is then set to 0 volts (e.g., through connection 270 inconjunction with pre-charge word-line 208-chg, or through connection280, as illustrated in FIG. 3), if the addressed TFT is to beprogrammed; otherwise, the voltage on the active strip of the addressedTFT is set to 10 volts if it is to remain in its erased state (i.e.,program-inhibited). The global word-line associated with the addressedTFT is then raised to 20 volt, either in one step or in short-durationsteps of incrementally increasing voltages, starting at around 14 volts.Such incremental voltage steps reduce the electrical stress across theTFT and avoid overshooting the target threshold voltage. All otherglobal word-lines in the block are set at half-select 10 volt. Allactive strips on all planes that are not being addressed in the block,as well as all active strips within the addressed plane that are notindividually addressed, are also set at 10 volts, or may be floated.These active strips are strongly capacity-coupled to the localword-lines, which are at 10 volts, and thus float at close to 10 volts.Each of the incrementally higher voltage programming pulses is followedby a read cycle to determine if the addressed TFT has reached its targetthreshold voltage. When the target threshold voltage is reached, theactive strip voltage is raised to 10 volts (alternatively the strip isfloated, and rises close to 10 volts when all but the one addressedglobal word lines in the block are raised to 10 volt) to inhibit furtherprogramming, while the global word-line keeps programming otheraddressed strips on the same plane that have not yet attained theirtarget threshold voltages. This programming sequence terminates when alladdressed TFTs have been read-verified to be correctly programmed. WhenMLC is used, programming of the correct one of the multiple thresholdvoltage states can be accelerated by first pre-charging capacitor C ofall addressed active strips (see, e.g., through connection 270 andpre-charge word-line 208-chg of FIG. 3) to one of several voltages(e.g., 0, 1.5, 3.0, or 4.5 volts, when two bits of information are to bestored in each TFT). 20 volt pulses are then applied to the addressedglobal word-line, which expose the TFT to different effective tunnelingvoltages (i.e., 20, 18.5, 17, or 15.5 volts, respectively), resulting inthe correct one of the four threshold voltages being programmed in asingle course programming step. Thereafter, fine programming pulses maythen be applied at the individual TFT level.

Because of the intrinsic capacitance C of every active strip in theblock, all active strips on all planes in a block can have theirpre-charge voltage states set in place in advance of applying the highvoltage pulsing on the addressed global word-line. Consequently,concurrent programming of a great many TFTs can be achieved. Thereafter,individual read-verify, and where necessary, resetting properlyprogrammed active strips into program-inhibit mode can be carried out.Pre-charging is advantageous, as programming time is relatively long(e.g., around 100 microsecond) while pre-charging all capacitors C orread-verifying of addressed TFTs can be carried out over a time periodthat is more than 1,000 times shorter. Thus, it is advantageous toprogram as many TFTs as possible in a single global word-lineprogramming sequence.

Erase Operation

With some charge-trapping layers, erase is accomplished throughreverse-tunneling of the trapped charge, which can be rather slow (e.g.,may require tens of milliseconds of erase pulsing). Therefore, the eraseoperation is frequently implemented at the block level, often in thebackground. A typical block may be eight planes high with each planehaving 8,000 active strips each with 4,000 TFTs on either of its sides,for a total of half a billion TFTs in a block, so that a one-terabitchip includes approximately 1,000 such blocks, if two bits ofinformation are stored on each TFT. Ideally, block erase is carried outby applying around 20 volts to the P− sublayer 222 (see, e.g., FIG. 2b-1) of every active strip through connection 290 (see, e.g., FIG. 3)while holding all global word-lines in the block at 0 volts. Theduration of an erase pulse should be such that most TFTs are erased to aslight enhancement mode threshold voltage, between zero and one volts.Some TFTs may overshoot and be erased into depletion mode (i.e., havinga slightly negative threshold voltage). Such TFTs are required to beprogrammed into a slight enhancement mode threshold voltage subsequentto the termination of the erase pulses, as part of the erase command.

Alternatively, in lieu of V_(bb) applied to P− sublayer, sublayers 221and 223 on all active strips are raised to around 20 volts while holdingall global word-lines at zero volt for the duration of the erase pulse.This scheme requires that strip-select decoders (206-0, 206-1 in FIG. 2c) employ transistors that can withstand the 20 volts at their junctions.Alternatively, all but the addressed global word-line are held at zerovolts, while pulsing the addressed global word-line to −20 volts andholding all active strips in planes 202-0 through 202-7 at zero volts.This method erases only the X-Z slice of all TFTs touched by the oneaddressed global bit line.

Erase for the NROM TFT embodiment can be achieved using the conventionalNROM erase mechanism of band-to-band tunneling-induced hot holeinjection. To neutralize the charge of the trapped electrons: apply −5volts on the word-line, zero volts on source sublayer 221 and 5 volts ondrain sublayer 223.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modification within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

The invention claimed is:
 1. A NOR string array formed over a planarsurface of a semiconductor substrate, comprising a first NOR string anda second NOR string, each NOR string comprising thin-film storagetransistors formed along an active strip extending lengthwise along afirst direction parallel to the planar surface, wherein the thin filmstorage transistors in each NOR string share a pre-charge device, acommon drain terminal and a common source terminal, wherein a gateterminal of each storage transistor in the first NOR string iselectrically connected to a gate terminal of a corresponding storagetransistor in the second NOR string by a conductor that extendslengthwise along a second direction which is substantially perpendicularto the planar surface, and wherein the pre-charge device pre-charges thecommon source terminal to a voltage that is substantially held by virtueof a parasitic capacitance in the source terminal during a program,program-inhibit, reading or erasing operation on the NOR string, furthercomprising a third NOR string, wherein the first NOR string and thethird NOR string are formed on a plane parallel the planar surface,wherein one or more storage transistors on the third NOR string areprogrammed to have reference threshold voltages, which are compared tothreshold voltages of storage transistors in the first NOR string duringprogramming or read operations.
 2. The NOR string array of claim 1,wherein the active strip of the first NOR string and the active strip ofthe second NOR string are both formed on a plane parallel to the planarsurface.
 3. The NOR string array of claim 1, wherein the third NORstring provides a reference string leakage current source by setting allthe storage transistors of the third NOR string are in an off state. 4.The NOR string array of claim 1, further comprising a plurality ofreference NOR strings, wherein when the storage transistors of the firstNOR string are programmed according to a multibit scheme, the referenceNOR strings each provide a reference for a different state of themultibit scheme.
 5. The NOR string array of claim 1, wherein theconductor comprises one of N⁺ doped polysilicon, P⁺ doped polysilicon,and a refractory metal of a high work function with respect to silicondioxide.
 6. The NOR string array of claim 1, wherein the common drainterminal comprises a semiconductor layer and a metallic layer that is inelectrical contact with, and in substantial alignment lengthwise with,the semiconductor layer.
 7. The NOR string array of claim 1, whereineach active strip has a first side-edge and a second side-edge onopposite sides of the active strip, wherein an independently controlledNOR string is formed along each side-edge.
 8. The NOR string array ofclaim 1, further comprising a first global wiring which is in electricalcontact with the conductor and which extends lengthwise along a thirddirection that is substantially orthogonal to both the first and seconddirection.
 9. The NOR string array of claim 8, wherein support circuitryfor memory operations is formed on or in the semiconductor substrate andwherein the first global wiring conducts electrical signals from thesupport circuitry to the conductor.
 10. The NOR string array of claim 9,wherein the support circuitry comprises voltage sources for providingthe configurations of voltages to be imposed on the terminals of eachstorage transistors, so as to effectuate program charging,program-inhibit charging, reading or erasing data stored in the storagetransistor.
 11. The NOR string array of claim 8, wherein the firstglobal wiring is formed between the semiconductor substrate and theactive strip of the first NOR string.
 12. The NOR string array of claim8, wherein the NOR string array further comprises a second global wiringformed above the active strip of the first NOR string, the first andsecond global wirings are in electrical contact with gate terminals ofdifferent storage transistors of the first NOR string.
 13. The NORstring array of claim 1, wherein the pre-charge device equalizes thevoltages of the common source terminal and the common drain terminal.14. The NOR string array of claim 12, wherein the common drain terminalis charged to one of several predetermined voltages prior to turning onthe pre-charge device, the predetermined voltage being selected from oneof: program, program-inhibit, reading and erasing data of the storagetransistors.
 15. The NOR string array of claim 1, wherein the commonsource terminal serves as a shared virtual ground reference and thecommon drain terminal serves as a common bit line for the storagetransistors in each NOR string.
 16. A NOR string array formed over aplanar surface of a semiconductor substrate, comprising a first NORstring and a second NOR string, each NOR string comprising thin-filmstorage transistors formed along an active strip extending lengthwisealong a first direction parallel to the planar surface, wherein the thinfilm storage transistors in each NOR string share a pre-charge device, acommon drain terminal and a common source terminal, wherein a gateterminal of each storage transistor in the first NOR string iselectrically connected to a gate terminal of a corresponding storagetransistor in the second NOR string by a conductor that extendslengthwise along a second direction which is substantially perpendicularto the planar surface, and wherein the pre-charge device pre-charges thecommon source terminal to a voltage that is substantially held by virtueof a parasitic capacitance in the source terminal during a program,program-inhibit, reading or erasing operation on the NOR string, whereinthe first NOR string and the second NOR string are formed on first andsecond planes parallel to the planar surface, respectively, the NORstring array further comprising one or more reference NOR strings oneach of the first and second planes, and wherein for each of thereference NOR strings, one or more storage transistors on the referenceNOR string are programmed to have reference threshold voltages, whichare compared during programming and read operations to thresholdvoltages of storage transistors in the corresponding NOR string on thesame plane as the reference NOR string.